Introduction to Chemistry
Grade 6 · Science · Study Guide
Discover the building blocks of matter, how atoms combine to form substances, and the difference between physical and chemical changes.
Chemistry is the study of matter — what it is made of, its properties, and how it changes. All matter is made of atoms, which are the smallest units of an element that retain the chemical properties of that element. The nucleus of an atom contains protons (positively charged) and neutrons (no charge), while electrons (negatively charged) orbit the nucleus in energy levels.
The Periodic Table organizes all known elements by atomic number (the number of protons). Elements in the same column (group) share similar chemical properties. When atoms of different elements combine chemically, they form compounds. Water (H2O) is a compound made of two hydrogen atoms and one oxygen atom.
Mixtures are combinations of two or more substances that are not chemically combined and can be separated by physical means. A saltwater solution is a mixture — you can evaporate the water to recover the salt. Compounds, on the other hand, can only be separated through chemical reactions.
Changes to matter are classified as physical or chemical. Physical changes alter the form or appearance of matter without changing its chemical composition. Cutting paper and melting ice are physical changes. Chemical changes produce new substances with different properties. Burning wood and rusting iron are chemical changes. Evidence of a chemical change includes color change, gas production, temperature change, formation of a precipitate, or emission of light.
Key terms
- Atom
- The smallest unit of an element that retains the chemical properties of that element; made of protons, neutrons, and electrons.
- Element
- A pure substance made of only one kind of atom; cannot be broken down into simpler substances by chemical means.
- Compound
- A substance formed when two or more elements are chemically combined in a fixed ratio.
- Mixture
- A combination of two or more substances that are not chemically combined and can be separated by physical means.
- Physical change
- A change that alters the form or appearance of matter without changing its chemical composition.
- Chemical change
- A change that produces one or more new substances with different properties from the original; also called a chemical reaction.
Flashcards
- Atom
- Smallest unit of an element with its chemical properties; contains protons, neutrons (nucleus) and electrons.
- Proton
- Positively charged particle in the nucleus; the number of protons = atomic number = element identity.
- Neutron
- Neutral particle in the nucleus; contributes to atomic mass; different numbers create isotopes.
- Electron
- Negatively charged particle orbiting the nucleus; involved in chemical bonding and reactions.
- Element
- Pure substance made of one type of atom; listed on the Periodic Table by atomic number.
- Compound
- Two or more elements chemically bonded in fixed ratios; has different properties than its component elements.
- Mixture
- Two or more substances combined but not chemically bonded; components retain their properties and can be separated.
- Physical change
- Change in form/shape/state without changing chemical identity; examples: cutting, melting, dissolving.
- Chemical change
- Produces new substances with new properties; examples: burning, rusting, cooking; usually irreversible.
Questions
What is the center of an atom called?
- Electron cloud
- Nucleus
- Orbital
- Ion
Show answer
Answer: Nucleus
The nucleus is the dense central core of an atom, containing protons and neutrons. Electrons orbit the nucleus in energy levels (electron shells) at a relatively large distance.
Melting ice is an example of a chemical change.
Show answer
Answer: False
Melting ice is a physical change — water (H2O) changes state from solid to liquid but its chemical composition remains exactly the same. No new substance is formed.
The number of ________ in the nucleus of an atom determines which element it is.
Show answer
Answer: protons
The atomic number of an element equals the number of protons in its nucleus. Every element has a unique atomic number; changing the number of protons creates a different element.
Which of the following is evidence that a chemical change has occurred?
- Water freezing into ice
- A piece of paper being cut
- Iron turning orange and flaky when exposed to moisture
- Sugar dissolving in warm water
Show answer
Answer: Iron turning orange and flaky when exposed to moisture
Rusting (oxidation of iron) is a chemical change because it produces a new substance — iron oxide — with different properties (color, texture, brittleness) than the original iron.
A compound can be separated into its component elements by physical means such as filtering.
Show answer
Answer: False
Compounds are chemically bonded and can only be separated into their component elements through chemical reactions. Physical methods like filtering or evaporation can only separate mixtures.
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